1. Field of the Invention
The invention relates to an improved method for the production of low cost CO2 amine-enriched sorbents that can be used in large-scale processes, and, more specifically, this invention relates to a method for the use of an aqueous system to incorporate the needed functionality group, an amine, onto the surfaces of oxidized solids.
2. Background of the Invention
Separation and capture of CO2 have been identified as a high-priority research topic in Department of Energy (DOE) reports. The costs of separation and capture, including compression to the required pressure for the sequestration step, estimated to comprise about three-fourths of the total cost of ocean or geologic sequestration. An improvement of the separation and capture of CO2 will reduce the total cost required for sequestration.
The most likely options for CO2 separation and capture include chemical absorption, physical and chemical adsorption, low-temperature distillation, gas-separation membranes, mineralization/biomineralization, and vegetation. The CO2 absorption process using aqueous amine solutions have been used for the removal of CO2 from gas streams in some industries. This process, based on the principles of chemical absorption of CO2 via monoethanolamine (MEA) or diethanolamine (DEA), is a potential technique for capturing greenhouse gas emissions from flue gas streams.
Wet chemical stripping of CO2 involves one or more reversible chemical reactions between CO2 and another substance such as MEA to produce a liquid species, such as a carbonate. Upon heating, the carbonate (heretofore isolated from the CO2 feed stream) breaks down to free CO2, with the original amine regenerated to react with additional CO2. An example of the process, with monoethanol amine, is: 
Typically, these amines, MEA and DEA, are used as 25 to 30 wt. % amine in aqueous solution. The amine solution enters the top of an absorption tower while the carbon dioxide containing gaseous stream is introduced at the bottom. During contact with the CO2-containing gaseous stream, the amine solution chemically absorbs the carbon dioxide from the gaseous stream. Desorption of the adsorbed carbon dioxide proceeds through a thermal regeneration process. Carbon dioxide and water emerge from the amine solution and the water is separated by condensing the water vapor in a heat exchanger. After regeneration the amine solution is recycled back to the absorption tower for additional carbon dioxide absorption.
Carbon dioxide capture and regeneration in the above-described manner
Carbon dioxide capture and regeneration in the above-described manner requires high temperatures or very low vacuum. The process is more complicated and costly. The steps outlined supra are energy intensive. Further, the amine solution has a limited lifetime due to degradation through oxidation of the amine. In addition, corrosion problems are usually observed for the aqueous amine process.
Several solid sorbents have recently been utilized to remove carbon dioxide from enclosed environments. Important considerations include the ability to regenerate an absorbent and the ease of its regeneration. Efforts have been made to reversibly adsorb CO2 on a silica gel first modified with amine. O. Leal, et al., Inorganica Chimica Acta, 240, 183-189, 1995. Surface modification occurs when the hydroxyl moieties of the silica gel surface bonds with chemical moieties. When the chemical moiety is 3-aminopropyltriethoxysilane, bonding occurs between the oxygen atoms of the ethoxy moieties and silicon atoms at the surface of the gel. It is this surface modification that facilitates adsorption of CO2 via the formation of carbamate species.
U.S. Pat. No. 3,491,031 awarded to Stoneburner on Jan. 20, 1970 discloses a method to create a CO2 sorbent by treating activated carbon with gaseous alcohol amines such as MEA. It emphasizes using a reaction between gaseous CO2 and an amine-enriched solid, but utilizes a wet-chemical stripping method employing MEA to remove the CO2 and regenerate the sorbent.
U.S. Pat. No. 5,886,061 awarded to Beckman on Mar. 23, 1999 discloses a method to create CO2 sorbents by the incorporation of amine groups into a polymer substrate or backbone.
U.S. Pat. No. 5,876,488 awarded to Birbara et al. on Mar. 2, 1999 discloses a method to create CO2 sorbents by dispersing aqueous amines in polymeric materials.
U.S. Pat. Nos. 5,620,940, 5,492,683 and 5,376,614, all awarded to Birbara et al. disclose methods to create CO2 sorbents by using amine-polyols on chemically inert supports.
U.S. Pat. No. 4,810,266 awarded to Zinnen, et al. on Mar. 7, 1989 discloses a method to create CO2 sorbents by treating carbonized molecular sieves with alcohol amines.
U.S. Pat. No. 4,531,953 awarded to Groose, et al. on Jul. 30, 1985 discloses a method to sequester toxic gases with solid sorbents which are produced by treating metal-impregnated activated carbons with anhydrous amines.
None of the aforementioned patents disclose a method using oxidized solids.
U.S. Pat. No. 4,320,011 awarded to Sato, et al. on Mar. 16, 1982 discloses a method to create an oil-containing waste water treatment material by treating activated carbons with aqueous amine solutions. It discloses treating graft-polymerized activated carbons with aqueous amines in order to obtain solid sorbents. It does not disclose a simple surface oxidation of the solid substrate.
A need exists in the art for a method to produce amine-enriched sorbents for the capture of CO2 from fluids. In particular, a need exists for a method to prepare such sorbents by simple chemical treatment of an already oxidized substrate surface with amine compounds. In addition, a need exists for a sorbent which retains CO2 over a range of operating temperatures. Finally, a need exists for a method which does not use expensive organic solvents and polymeric materials and in which the sorbent produced is thermally stable, and easily regenerated.
An object of the present invention is to provide a method for synthesizing amine-enriched sorbents that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a new method for synthesizing amine-enriched sorbents. A feature of the invention is the elimination of the need for organic solvents and polymeric materials for the preparation of CO2 capture systems. An advantage is that the new method is inexpensive.
Still another object of the present invention is to provide a method for creating many different CO2 absorbing materials. A feature of the invention is the suitability of a large selection of solid substrates and amine-based compounds as absorbing material building blocks. An advantage of the invention is that a greater range of CO2 absorbing capabilities can be provided.
Yet another object of the present invention is to provide a method which produces sorbents which absorb over a range of temperatures. A feature of the invention is that the sorbents provided by this method adsorb CO2 from 20xc2x0 C. to 80xc2x0 C. via a combination of both physical and chemical adsorption processes. An advantage of the invention is that these sorbents can adsorb at temperatures above normal ambient temperatures, and well above 30xc2x0 C.
Still another object of the present invention is to provide a method which gives a sorbent which is easily regenerated. A feature of the invention is that regeneration of the sorbent can be accomplished in an anhydrous environment or by heating above 80xc2x0 C. An advantage of the invention is that the regeneration process is inexpensive.
Yet another object of the present invention is to provide a method which gives sorbents which are thermally stable. A feature of the invention is that the sorbents provided by this method can be heated to temperatures above 100xc2x0 C. with little or no degradation. An advantage of the invention is that these sorbents have a longer life span of usefulness which results in lower costs.
Briefly, the invention provides a process for producing an amine-enriched sorbent which comprises treating an oxidized surface with base, and; treating the surface with a substituted amine salt.
The invention also provides a process for producing an amine-enriched sorbent which comprises treating a surface with a substituted amine salt in the presence of a catalyst.